PSK-1/TNC-NB96 Combo KISS mode

Maybe I'm losing it, but I can't get my TNC-NB96 to go into KISS mode. The
firmware is V5.0. I try KISS ON or MKISS, but I get '?unknown command' back.
Other functions seem to work perfectly.
Anybody got any ideas?
73s, Howard G6LVB
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